1. Field of the Invention
The present invention relates to a fan with a direct current motor where the speed of the rotary blade is controlled depending on the ambient temperature.
2. Brief Description of the Background of the Invention Including Prior Art
A direct current fan with control means employing temperature as the sensed parameter for the control function is taught in U.S. Pat. No. 4,426,604 issued Jan. 17, 1984 entitled DIRECT CURRENT FAN WITH CONTROL MEANS. The circuit employs a sensor for sensing the speed of the motor, which is amplified in an amplifier and the resulting signal is then fed to an r.p.m. comparator. FIG. 2 discloses a positive temperature coefficient (PTC) resistor used to increase the speed upon increase in temperature. The PTC resistor is connected between one power input terminal and a series connection of two resistors and comparator terminals to the other power input. The comparator is connected to a sensor for the speed of the motor. This circuit is complicated by the use of two comparators, an amplifier and the speed control sensor and provides for a limitation of the size of the current fed to the windings of the motor.
U.S. Pat. No. 2,991,405 to Carlson teaches a transistorized motor control system. A thermistor 42 controls via transistors T2, T3 and T4 a motor 2. This circuit does not provide for any minimum stabilized voltage to be fed to the motor.
An electronic thermostat is taught in German Patent No. 2,331,022. A rectifying circuit is shown where the motor is switched between two rotation speeds and standstill.
A circuit for controlling the rotary speed of an electric motor driving a fan is taught in German Patent No. 2,845,437. Relays are provided for switching between the rotary speed in each case followed by on-off switches.
U.S. Pat. No. 4,286,198 to de Valroger teaches a direct current motor unit without commutator. This reference teaches the use of a protective means for a Darlington transistor, but no temperature control is provided at all. This reference also shows a bridge-circuit by numerals 41, 42, 51, 30, 40, 44.
A motor of the kind useful in the context of the present invention is shown in U.S. Pat. No. 4,030,005 by way of example. However, other D.C. motors known in the art can also be employed to be connected to the output power of the circuit of the present invention.
At present there is frequently a requirement associated with the ventilation or, respectively, cooling of electronic equipment for noise reasons that the fan provides the power in fact required for the cooling, that is the fan reduces its rotation speed in case of a lower power requirement in order to achieve a minimum noise development.
The solution of a similar problem is disclosed in German patent application No. P 3,024,613.0, where however, the direct current motor is connected to a control circuit for limiting the current.
In contrast, the present invention relates to a control of the rotation speed or, respectively, voltage of a fan or, respectively, of its immediately directly driving axial direct current motor with electronic commutation depending on the ambient temperature.
The operation of direct current motors with a voltage stabilization circuit at a direct current power grid is known. Furthermore, it is of concern to obtain the mentioned functional dependence on the ambient temperature and in particular under special boundary conditions.
The boundary conditions can be preset from the outside arbitrarily or by outer circumstances. They can also become necessary by a special kind of driving motor, or by a special kind of its operating circuit. For example an operating circuit for a D.C. motor is set forth in German Patent Application Laid Open DE-OS No. 2,419,432, where based on a capacitance a so-called block protection is achieved, that is the motor does not accept any current in a braked or retarded state. Such a block protection by a capacitance however, in certain circumstances effects a voltage change occurring too slowly in connection with a temperature change or in cases, where the voltage rises very slowly starting from values too low, the start-up of a motor with such temperature control is prevented. The problem exists also with other block protection based on electrodynamic action, for example in case of such a block protection where decoupled voltage values are decoupled by rotation of the rotor magnet via diode elements. It may be desirable to allow the fan voltage according to FIG. 2 or its rotation speed to change linearly via the temperature, for example proportional to the natural zero degree point in centigrade as is shown approximately in FIG. 2. If then the rotation speed is lowered by cooling and correspondingly the operating voltage is lowered to small values, and then a slow temperature increase or, respectively, a voltage increase from too small values starts again, then under certain circumstances the fan would not run up any longer corresponding to the characteristic curve branch A of FIG. 2, since the voltage change du/dt would change too slowly of the fan motor protected by the capacitance block therefore, here a minimum voltage of 16 volts is fixed corresponding to a rotation speed of 2000 rotations per minute, which is independent of temperature, that is which is constant and which is represented by the characteristic curve branch B and this characteristic curve branch B is now to be realized. However, if the temperature increases beyond the characteristic curve intersection point of the branches A and B, or if a voltage higher than corresponding to this intersection point is applied, then the circuit is to operate automatically according to branch A.